1. Field of the Invention
This invention relates to semiconductor devices and, in particular, dielectrically isolated semiconductor devices.
2. Art Background
Electronic devices employing dielectric isolation offer advantages both for high-voltage applications and for applications requiring high reliability signal processing. These advantages are derived from the structure of such devices. In particular, a single crystal silicon active region or regions are separated from a common substrate by a region of dielectrically insulating material, e.g., silicon oxide. (The active region is the region where electronic devices such as transistors, diodes, and capacitors are to be or have been formed.) The dielectrically isolating material(s) provides a sufficient barrier to processes such as avalanche breakdown so that high voltages in one active region, e.g., voltages above 30 V, do not cause interaction with a second active region on the same substrate. Additionally, the dielectric insulator prevents carriers formed by interaction of cosmic radiation with the substrate from diffusing into an isolated active region and causing inaccuracies in information processed by the devices in that active region.
The use of a device quality silicon region on a bulk dielectric material is also advantageous in other applications such as for flat panel displays. In such applications device quality silicon overlying a dielectric material such as fused quartz is employed. The fused quartz is clear and is chosen to allow viewing of the display medium which generally overlies the silicon layer. The thin silicon layer is patterned to further enchance the viewability of the display medium and has a device or devices formed in it which control the operation of this medium.
Many procedures have been developed for producing a dielectrically isolated region of device quality silicon, i.e., silicon having a mobility of at least 100 cm.sup.2 /V.multidot.second. One general type of procedure for fabricating dielectrically isolated structures involves traversing a molten zone across a polycrystalline silicon layer that has been deposited on a dielectric material such as silicon oxide. Generally, the molten zone is initially formed at a region of single crystal silicon adjoining the polycrystalline silicon. The molten zone is then traversed through the polycrystalline silicon to convert the region which overlies the dielectric material into device quality silicon. In one particular technique involving molten zone tranversal, a body having a dielectric material that is overlaid by a poly crystalline silicon layer is generally heated to a temperature of approximately 1100 degrees C. A strip of graphite having a width of approximately 1 mm and a length commensurate with the size of the substrate is resistively heated, positioned above the polycrystalline layer to form a melt zone, and translated to propagate this melt zone across the polycrystalline silicon. Without the supplemental heating, i.e., heating of the polycrystalline silicon region to 1100 degrees C., the graphite strip heater will not cause melting under practical conditions. Generally, by this graphite strip heater method which involves high temperature supplemental heating, silicon region having an average size of approximately 2 cm by 2 cm and typical mobilities of approximately 520 cm.sup.2 /V.multidot.second are produced. (See E. W. Maby et al, Electon Device Letters, EDL-2, 241 (1981) for a general description of this procedure and a discussion of the properties achieved.) Despite the acceptable quality of the resulting silicon, the process requires additional capping layers overlying the silicon layer and extremely strict control of processing parameters such as substrate temperature to prevent beading of the silicon layer with its resulting destruction. (See M. W. Geis et al, Applied Physics Letters, 40, 158 (1982).) Even with strict control, relatively uniform results are not achieved from sample run to sample run.
Lasers have also been employed to traverse a molten zone across a polycrystalline or amorphous silicon region, i.e., a non-single crystal silicon region. Available lasers have sufficient power to produce the melt zone without supplemental heating. Thus, supplemental heating is not essential. However, nominal supplemental heating--temperatures typically at or below 350 degrees C.--in laser techniques has been employed to reduce the power requirements on the laser and to achieve what is considered advantageous size melts, e.g., essentially round melts approximately 30 .mu.m in diameter. Since low supplemental temperatures have been considered an attribute to laser processing, use of temperatures above about 350 degrees C. have been generally avoided. (See J. F. Gibbons, International Conference Solid State Devices, Tokyo, 1979.) Although the mobilities of the resulting silicon are typically on the order of 100 cm.sup.2 /V. seconds, the mobility often varies substantially across the layer being processed.
Even though many techniques have been developed for producing dielectrically isolated silicon and even though many of these techniques have been found to be useful, it is always desiralbe to improve the mobility of the dielectrically isolated silicon region obtained and most significantly to dimish the difficulty involved in consistently obtaining regions with excellent and relatively uniform mobilities, i.e., mobilities varying no more than 25 percent over an area equal to at least 90 percent of the treated area.